A surface-emission laser diode of vertical-cavity surface-emission laser (VCSEL) structure is a laser diode that emits light in a direction perpendicular to a substrate and draws attention these days in view of its advantageous features of low cost, low power consumption and compact size as compared with the laser diode of edge-emission structure that emits light in a direction parallel to the substrate. Further, such a surface-emission laser diode of VCSEL structure has high performance and is suitable for constructing a two-dimensional device.
The laser diode of VCSEL structure can be used for example for the optical source used for optical writing in printers, or the like, the optical source used for optical writing in optical disk apparatuses (oscillation wavelength: 780 nm; 850 nm), and the optical source for optical transmission systems such as LAN (local area network) that uses optical fibers (oscillation wavelength: 1.3 μm; 1.5 μm). Further, it is thought promising to use such a surface-emission laser diode for the optical source of optical transmission between circuit boards, between devices inside a circuit board, between chips forming an LSI (large-scale integrated circuit), and between the device elements inside an LSI.
In the application area of such VCSELs, there frequently arises the demand that the output light thereof forms the light of single fundamental mode and has a constant polarization mode. For example, it is desired to use an optical source of single wavelength and constant polarization mode in optical writing systems, in which there is a need for precise and complex control of optical path for focusing the output optical beam. Further, in the case of optical transmission systems, too, it is preferable to use an optical source of single wavelength and constant polarization mode for transmitting signals over long distance with low noise while suppressing mode competition.
Patent Reference 1 discloses a VCSEL having a fixed polarization direction. Further, Patent Reference 2 discloses a surface-emission laser diode capable of controlling the polarization direction of the laser light reproducibly in a specific direction. Further, Patent Reference 3 discloses a surface-emission laser diode in which two or more peripheral high-resistance layers are interposed between a first reflection mirror and a second reflection mirror with respective, different proportions of resistance increase. Further, Non-Patent Reference 1 discloses generation of optical gain anisotropy by using an inclined substrate.    Patent Reference 1 Japanese Laid-Open Patent Application 9-172218    Patent Reference 2 Japanese Laid-Open Patent Application 2006-13366    Patent Reference 3 Japanese Laid-Open Patent Application 11-307882    Non-Patent Reference 1 Iga, K., Koyama, F. (ed.) Fundamentals and Application of Surface-emission Laser, (in Japanese) Kyoritsu Publishing K.K.